Conventionally, so-called general purpose polystyrene (GPPS)-based resin such as styrene homopolymer which is excellent in tensile strength, heat resistance, light resistance, formability and surface luster, and high impact polystyrene (HIPS) wherein rubber such as SBR and BR is blended with GPPS to reduce its brittleness, have been often used for food containers such as beverage containers, yoghurt containers, portion containers, cup noodle containers, and for synthetic resin containers to be filled with disposable medical supplies, etc. As a cap material to be put together and seal an opening of such polystyrene-based resin container, an aluminum laminated body wherein an aluminum foil is used as a base material, a sealant layer, etc., for adhering to the container are provided on its surface, is used. An aluminum cap, which is produced by forming a small piece of aluminum cap material being punched out into an extensive form of the cap from the aluminum laminated body thus described into a shape with a skirt by folding back its end, and with which an opening of a container is sealed, is commonly used because of the following reasons: it has an excellent sealing property, a peel resistant property, and excellent stability at the time of peeling; when it is fed to the opening of the container, it shows low level of adhesion caused by static electricity, and feedability of sheet is good. In addition, the aluminum cap has a so-called shape retaining property which is a property to maintain a folded and deformed shape when a skirt, which has been formed by folding back its peripheral part, is provided. Therefore, when drinking a filled beverage directly from the container, the state wherein the part in the vicinity of the opening of the container which comes into contact with the mouth is covered with the end of the cap, is well retained, and an area in the vicinity of the opening of the container can be prevented from getting dirty. It is thus hygienically excellent, and excellent in appearance as well. Therefore, it has been preferably used.
Further, as an alternative for the conventional aluminum cap thus described, a cap made of synthetic resin, has been proposed. For example, the followings have been proposed: a cap material manufactured by punching out a laminated material wherein a sealant layer is provided on the lower face of a laminated base material which has been constructed by laminating a heat-resistant film on both sides of a base material of a co-extruded film comprising a central layer constituted of high-density polyethylene and polypropylene-based polymer, and a coat layer constituted of high-density polyethylene, which is provided on both sides of the central layer, into a given shape (for example, see Patent Document 1); and a container for liquid comprising a container body having a mouth part, and a cap which closes the mouth part, wherein the whole of the container body and the cap is made of a synthetic resin (for example, see Patent Document 2). In addition, a resin sheet for cold forming that can be formed by cold-draw-forming has been proposed as a cap material of a packaging container (for example, see Patent Document 3).
Patent-Document 1: Japanese Laid-Open Patent Application No. 11-10810
Patent-Document 2: Japanese Laid-Open Patent Application No. 2002-225902
Patent-Document 3: Japanese Laid-Open Patent Application No. 2004-74794